The present invention relates to an apparatus and a method for determining the center of annual rings of a wood block. More specifically, the invention relates to an apparatus and a method for determining or specifying the annual ring center in an image of an end of a wood block.
In manufacturing laminated wood products such as plywood and laminated veneer lumber (LVL), wood veneer is used which is produced by cutting or peeling a generally cylindrical wood block by a veneer knife. The veneer knife is fed into the wood block rotating on its spin axis from the peripheral surface toward that spin axis of the wood block. Such cutting is done by a machine called a rotary veneer lathe.
FIG. 8 is a schematic diagram showing a typical rotary veneer lathe. The rotary veneer lathe includes a pair of spindles 101 (only one spindle being shown in the drawing) each having at the inner end thereof a chuck for supporting a wood block 200 at the opposite ends thereof so as to rotatably holds the wood block 200. The rotary veneer lathe further includes a knife carriage 102 having a veneer peeling knife and movable toward the spindles 101 the spin axis of the wood block 200, as indicated by arrow A, for a predetermined distance for each turn of the block 200. The feed rate of the knife carriage 102, or the distance moved by the knife carriage 102 for each turn of the block 200, is controlled by an electrical signal generated in accordance with the rotation of the spindles 101.
The veneer peeling knife 103 is fixedly mounted on the knife carriage 102 at a position adjacent to the top of the knife carriage 102 for movement therewith. Reference numeral 104 designates a plurality of discs each having at the outer periphery thereof a number of tooth-like projections and driven to rotate in arrow direction B. These discs 104 are juxtaposed at a predetermined spaced interval in the direction that is perpendicular to the direction A in which the knife carriage 102 is moved. A nose bar 105 is provided between any two adjacent discs 104 for pressing the outer periphery of the wood block 200 at a position immediately above the cutting edge of the veneer peeling knife 103.
In such a rotary veneer lathe, the wood block 200 is supported at the opposite ends thereof by the spindles 101 and driven to rotate in arrow direction C by pressingly engaging the discs 104 rotating in arrow direction B at the piercing projections thereof with the outer periphery of the wood block 200. Moving the knife carriage 102 toward the spin axis of the wood block 200 at a controlled feed rate in arrow direction A, a sheet of veneer 210 having a predetermined thickness is peeled by the knife 103 from the wood block 200.
The wood block 200 that is prepared by cross-cutting a natural log to the desired length does not have a truly cylindrical shape. In order to produce veneer 210 from such wood block 200 with as high yield as possible, it has been general practiced to hold by the spindles 101 the wood block 200 at an optimum spins axis that corresponds to the center of gravity such as 202 (FIG. 9) of the block ends. Holding a wood block at such center of gravity as the spin axis for the peeling operation, however, problems occur in peeling particularly softwoods or coniferous woods, such as sugi (Japanese cedar).
Referring to FIG. 9 showing an end of a wood block 200 of a softwood and a pattern of annual rings, the wood block 200 has at the core thereof juvenile wood extending radially outward from the center pith 201 of the wood block for an extent corresponding to about 15 years of tree age, as indicated by shaded area in FIG. 9. The juvenile wood has characteristics that are different from those of the radially outer mature wood of the same wood block 200. For example, as compared to the mature wood, the juvenile wood shrinks and swells to a larger extend with a change of moisture content and is lower in strength than the mature wood of the same wood block.
If such softwood block 200 is rotated about the center of gravity 202 as the spin axis by a rotary lathe as shown in FIG. 8 for veneer production, veneer containing no juvenile wood is produced from the outer mature wood, while veneer containing both juvenile wood and mature wood (such veneer being referred to as mixed veneer) is produced from wood adjacent to and around the center of gravity 202. In the aforementioned LVL having a plurality of veneer sheets laminated together with the grain thereof oriented generally in the same direction, if a sheet of mixed veneer is present in either of the opposite outermost layers of LVL, the resulting lumber tend to be easily broken when it is subjected to tension or compression by bending. The problem may be avoided by using the mixed veneer sheet in layers of LVL other the outermost layers.
Peeling veneer by rotating the wood block 200 about its center of gravity 202 produces a relatively large volume of mixed veneer. In order to avoid the use of mixed veneer in the outermost layers of LVL for solving the above problem, laborious sorting of mixed veneer from normal veneer containing no juvenile wood may have to be performed. In view of the relatively large amount of mixed veneer produced, a situation may occur where mixed veneer cannot help but be used in the outermost layers of LVL. Thus, the problem remains unsolved.
On the other hand, peeling veneer from a hardwood by rotating the wood block about its center of gravity, such as 202, creates a problem that is different from the above problem associated with softwood. Generally, wood has rays or ray tissue (not shown in FIG. 9) extending radially outward that is important for regulating the radial strength of the wood. Ray tissue is hard particularly in hardwood. FIG. 10A shows in cross-section a veneer sheet peeled from, a block of hardwood rotated about the center of gravity. The ray tissue 211 is indicated by oblique line in FIG. 10A. As shown in FIG. 10A, most of the radial ray tissue 211 in the veneer sheet 210 extend obliquely with respect to the opposite surfaces of the veneer sheet 210.
If a plurality of veneer sheets such as 210 laid one on another for manufacturing LVL is pressed by a pair of heat plates and receives over the entire opposite surfaces thereof pressing forces as indicated by opposing arrows in FIG. 10A, the oblique ray tissue 211 is further inclines with respect to the veneer sheet surfaces, with the result that the veneer sheets are excessively deformed plastically in the thickness direction. The LVL made of such deformed veneer sheets is deficient in the desired thickness and, therefore, will be rejected as defective.
In order to solve the above problems encountered in peeling veneer from wood block, Kokoku (Japanese Examined Patent Publication) No. S62-45803 discloses a method of peeling veneer. The Publication proposes that a wood block, whose pith such as 202 corresponding to the annual ring center fails to coincide with the center of gravity such as 201 in FIG. 9, should be rotated about the latter annual ring center in peeling veneer from the block. By so peeling veneer from a softwood, the resulting veneer consists of veneer peeled from juvenile wood and veneer peeled from the rest of the wood block. Using the sheets of former veneer in inner layers and using the sheets of latter veneer in outer layers of LVL, the problem with the strength against bending can be solved successfully.
In the case of peeling veneer from hardwood by using the method according to the above Publication, ray tissue, such as 211 in FIG. 10B, in the resulting veneer extend substantially perpendicularly with respect to the opposite surfaces of the veneer sheet, as shown in FIG. 10B. Thus, veneer sheet is less prone to plastic deformation due to the pressing force because the hard ray tissue serves to support the radial strength of wood. Therefore, LVL made of such veneer sheets is prevented from having a defect due to the lack of thickness.
According to the method proposed by the above Publication, however, a workman needs to observe the ends of each wood block carefully to locate the position of the pith or the annual ring center with the naked eyes each time a wood block is to be mounted on a rotary veneer lathe for determining the spin axis of the block for veneer peeling operation. This requires the workman to do a lot of troublesome and hence inefficient work, including observing the block end, locating the annual ring center, setting spindles to the located annual ring center for chucking the wood block.
An object of the present invention, which has been made in light of the above-described problems, is to provide a method and an apparatus for automatically determining the annual ring center of a wood block, thereby improving the working efficiency in veneer peeling operation by a rotary veneer lathe.